Breathers for liquid operated heat exchangers

ABSTRACT

Heat exchangers for recooling the cooler liquid such as cooling water of steam operated power plants have to be provided with means by which insoluble gases such as air in the cooling water may reliably be removed therefrom. Moreover, with cooling systems of power plants where the recooling of the cooling water is obtained by means of air (&#39;&#39;&#39;&#39;air condensation&#39;&#39;&#39;&#39;), care has to be taken that, in addition to removing insoluble gases from the cooler liquid, air may quickly enter into the heat exchangers when the latter are emptied for operational reasons whatever such as danger of frost. This is obtained by employing a breather having an upright vent pipe which is high enough to accommodate a column of cooling water which, even in case of highest hydrostatic pressure, stays below the upper orifice of the vent pipe. Freezing in of the water columh is prevented by a jacket surrounding the vent pipe and filled with an anti-freeze liquid. In operation, insoluble gases may freely pass the vent pipe the cross-sectional area of which is large enough to permit suitable amounts of air to enter the heat exchanger when the latter is being emptied. A further advantage consists in that a plurality of heat exchangers may be serviced by a single vent pipe or breather.

[ July 23, 1974 BREATHERS FOR LIQUID OPERATED HEAT EXCHANGERS [75] Inventors: Laszl Heller; Lszl Forgo; Janos Bodas, all of Budapest, Hungary [73] Assignee: Transelektro Magyar Villamossagi Kulkereskedelmi Vallalat, Budapest,

Hungary [22] Filed: Oct. 2, 1972 [21] Appl. No.: 294,128

[30] Foreign Application Priority Data Oct. 5, 1971 Hungary .EE 1963 52 US. 01.... 165/111, 137/334, 137/341, 138/34, 165/134, 165/138, 261/D1G. 11 51 1111.01. F28b l/06 [58] Field 61 Search 138/32, 33, 34,114, 143,; 138/149;165/111,112,113,134;137l334,: 340,341:

56 References Cited UNITED STATES PATENTS 170,305 11/1875 Sampson 138/32 1,327,507 1/1920 Walther... 138/33. 1,863,437 6/1932 C ollier 165 134 x 2,247,056 6/1941 Howard 165/112 X 3,231,013 1/1966 Heller et a1. 165/111 X 3,660,980 5/1972 Knirsch et a1 165/113 X 3,706,872 12/1972 Trabilcy 137/341'X Primary Examiner-Albert W. Davis, Jr. Assistant Examiner-S. J.. Richter & Kaplan upright accommodate a column of cooling water which, even -in case of highest hydrostatic pressure, stays below the upper orifice of the vent pipe.

[57] ABSTRACT Heat exchangers for recooling the cooler liquid such as cooling water of steam operated power plants have to be provided with means by which insoluble gases such as air in the cooling water may reliably be removed therefrom. Moreover, with cooling systems of power plants where the recooling of the cooling water is obtained by means of air (air condensation), care' has to be taken that, in addition to removing insoluble gases from the cooler liquid, air may quickly enter into the heat exchangers when the latter are emptied for operational reasons whatever such as danger of frost.

This isobtained by employing a breather having an vent pipe which is high enough to pipe the cross-sectional area of which is large enough to permit suitable amounts of air to enter the heat exchanger when the latter is being emptied.

A further advantage consists in that a plurality of heat exchangers may be serviced by a single vent pipe or breather.

6 Claims, 7 Drawing Figures PATENTEU JUL2 3 I974 SHEEI 3 BF 3 "Fig.4

1 BREATHERS FOR LIQUID OPERATED HEAT EXCHANGERS This invention relates to breathers for liquid operated heat exchangers.

When such heat exchangers are filled up with a heat exchanger liquid such as water, care has to be taken that air and gases potentially present in the heat exchanger may withdraw therefrom. Similarly, when the heat exchanger is being emptied, air has to be permitted to penetrate into the chamber from which the water should withdraw. If the heat exchanger liquid contains unsoluble gases such as air, the latter have to be removed even in operation of the heat exchanger.

If a considerable plurality of heat exchangers is employed or else the heat exchangers have to be filled up and emptied frequently, preferably a breather will be employed which operates automatically without the need of external manipulation. If water is employed as heat exchanger liquid and there is a danger of frost, care has to be taken that the breather should operate reliably even at dangerously low temperatures. If the heat exchanger is quickly emptied because there is danger of frost, air has to be permitted to penetrate quickly into the chamber from which the water has to withdraw. Such requirements have to be met, e.g., by the heat exchangers of the known air condensation equipments of power plants with which recooling of the cooling liquid usually water of the power plant is obtained by means of air. e In compliance with the main feature of the present invention, this is obtained by employing breathers for ranged for being filled with an anti-freeze liquid and to permit a natural flow upon warming up of the antifreeze liquid along the whole height of the vent pipe. The collecting pipe conduit is connected with the inlets and outlets of the heat exchangers and is arranged so as to permit a flow therein due to a pressure difference prevailing between the heat exchanger inlets and outlets.

Furthermore, a reduced portion can be provided between a lower portion and an upper portion of the vent pipe at a level where the level of the heat exchanger liquid associated with normal operation of the heat exchangers assumes a position in the reduced portion of the vent pipe. The liquid level in the vent pipe supports a ball the specific weight of which is smaller than the specific weight of the heat exchanger liquid. The lower and upper portions of the vent pipe have greater diameters than the reduced portion therebetween, and are provided with grids the mesh size of which is smaller than the diameter of the ball.

Instead of a ball, however, a granular material may be employed in which case the lower portion and the upper portion of the vent pipe communicate with one another through the middle portion of reduced diameter and through vessels having porous walls. These vessels enclose a chamber with one another and with the reduced portion of the vent pipe. The chamber, in turn,

liquid operated heat exchangers of the type'having, in

combination, an upright vent pipe connectable to an air outlet of a heat exchanger and opening with its upper orifice into the ambiency, the vertical height of said vent pipe being selected so that a liquid level associated with the highest hydrostatic pressure stays below said upper orifice, a jacket surrounding said vent pipe and enclosing a chamber therewith adapted to be filled with an anti-freeze liquid and to permit a natural flow thereof upon its warming up along the whole height of said ventpipe.

In a preferred embodiment, a portion of the vent pipe is arranged for contacting, on the one hand, with the anti-freeze liquid between the jacket and the vent pipe and, on the other hand, with a heat exchanger liquid in the vent pipe. This portion forms a heat transmittance surface adapted to transmit an amount of heat from the heat exchanger liquid into the anti-freeze liquid which is needed for maintaining the latter at a temperature above its freezing point.

Preferably, a heat source will be provided in the chamber between the vent pipe and the jacket to which external energy is supplied by suitable means.

It is possible to employ a single breather of the aforesaid type in case of a plurality of heat exchangers associated therewith. Then the vent pipe of the breather is connectable to a common collecting pipe conduit connecting the air outlets of all heat exchangers. The upright vent pipe being connected to an intermediate portion of the collecting pipe conduit and opening with its upper orifice into the ambiency, the vertical height of the vent pipe being selected so that the level of liuqid associated with the highest hydrostatic pressure stays below the aforesaid upper orifice. A jacket surrounds" the vent pipe and encloses a chamber therewith arcomprises the granular material referred to above which will float on the liquid level in the vent pipe. The granular material has a specific weight which is smaller than the specific weight of the heat exchanger liquid. Moreover, it has a grain size which is greater than the size of pore of the vessel walls. The volume of the vessels is selected so that the cross-sectional area of the vessels leftfree after accommodating the granular material in the chamber formed by the vessels and the re duced portion is substantially equal to the crosssectional area of the reduced portion.

Further details of the invention will be described by taking reference to the accompanying drawings which show, by way of example, various embodiments of the breather according to the invention, and in which:

FIG. 1 is the connection diagram of a portion of a power plant working with recooling of" the cooling water by means of air known per se.

FIG. 2 shows a longitudinal sectional view of an exemplified breather according to the invention.

' FIG. 3 illustrates the connection diagram similar to that of FIG. 1 of another exemplified embodiment,

working with a plurality of heat exchangers.

FIG. 3a represents another exemplified embodiment of a detail.

FIG. 4 is a longitudinal sectional view of still another exemplified embodiment of the breather according to the invention.

FIG. 5 shows a still further exemplified embodiment similar to that shown in FIG. 4.

FIG. 5a illustrates a detail of FIG. 5 on a larger scale.

Same reference characters partly designate similar details throughout the drawings.

With the per se known plant shown in FIG. 1, recooling of the cooling water of the system is obtained by condensation by means of air (air condensation). The cooling water is supplied by a circulating pump 32 from a mixing condenser 31 into heat exchangers such as heat exchanger 33. The cooling water reaches the 3 heat exchanger 33 through valve 41 at a first subchamber of water chamber 34 from which it flows into cooling pipes of the heat exchanger proper in an upward direction as suggested by an arrow 47. In an upper water chamber 35 the flow direction of the cooling water is reversed so that it flows in the direction of arrow 37 downwards through further cooling pipes of the heat exchanger 33 into an other subchamber of the lower water chamber 34 wherefrom it flows back into the mixing condenser 31 through a valve 40, a reflux conduit 46, and a throttle means 39. The latter serves for adjusting the pressure prevailing in the above described cooling circuit.

The number of heat exchangers employed in air condensation cooling systems is relatively high and the heat exchangers are arranged in parallel connection. For being de-aerated or vented, the heat exchangers of known systems are provided each with vent valves 38 operated by swimmers, each heat exchanger 33 being provided with a separate vent valve 38.

. Air needed for cooling the heat exchanger 33 and its follow members is being sucked through a cooling tower 36which operates either with natural draft or with a ventilator. When the plant is out of operation, the heat exchanger 33 is filled with air, its valves 40 and 41 being closed. Upon opening the valves 40 and 41, the water flows into the lower water chamber 34 of heat exchanger 33 ,and air is dispelled by the rising water through vent valve 38. When the heat exchanger 33 has been filled with water, the cooling water begins to circulate in the aforesaid circulation line 46. The

pressure prevailing in the upper water chamber will beadjusted bythe throttle means 39 in such a manner that it exceeds the atmospheric value by some tenths of atmosphere. Thus, it is prevented that air penetrates somewhere into the cooling system.

A substantially increased pressure will, however, prevail in the water chamber 35 when the filling of the plant takesplace in such a manner that, in the beginning, those pipes of the heat exchanger 33 will be filled with water in which the water is flowing in a downward direction 37, and the pipes of the heat exchanger 33 in which the operational flowdirection of the cooling water points upwards in the direction of 'arrow 47, will be filledup subsequently. Thus, it is prevented that air is retained in the pipes with in operation upwards directedwater flow.

Filling up is carried out in the following manner:

First, valve is opened and, after the lapse of some I seconds, also valve 44 is opened. Thereby, first the changers, smaller than the pressure prevailing in their upper water chambers. If the pressure prevailing in the upper water chambers of the already operated heat exchangers exceeded the ambient pressure only slightly, then, upon opening the valve 40, the water would not rise in the reflux pipes of the heat exchanger 33 into the upper water chamber 35. Therefore, with filling up a considerably higher operational pressure has to prevail in the system than the pressure prevailing in the upper water chamber of the heat exchangers which is obtained by an increased throttling of the reflux pipe conduit 46 by throttle means 39. The vent valve 38 has to let air or insoluble gases, penetrated into the heat exchangers, to escape therefrom.

Disconnecting of heat exchangers due to operational requirements or to danger of frost is carried out by closing the valves 40 and 41 and, simultaneously, opening drain valves 42 and 43. Then, the cooling water is being discharged from the heat exchanger 33 into a drain vessel 44. Such emptying, however, is only possible if the ambient airis permitted to occupy the place of water in the heat exchangers. Such change is-rendered possible by the vent valve 38 throughwhich air pipes with in operation downwards directed flow 37 and then the pipes with in operation upwards directed flow 47 will be filled with water. This means that the reflux side 37 of the heat exchanger becomes filled up with water sooner than the side with forward flow direction 47. The dispelled airescapes through the vent valve 38. Then, circulation of the cooling water through-the heat exchanger 33 begins. If other heat exchangers such as heat exchanger 45 have already been in operation prior to filling up of heat exchanger 33 and heat exchangers of the same group, respectively, a substantially higher pressure has to prevail in the upper water chambers of heat exchanger 45 and its fellow heat exchangers while the heat exchanger 33 with its group members is being filled up. Viz, the static pressure prevailing in the reflux pipe conduit 46 is, by the water sideresistance of the reflux pipes of the heat ex may enter the water chamber of the heat exchanger.

The breather according to the invention is destined to permit such replacement by hydraulic means.

As goes forth from FIG. 2, the upper water chamber 1 of a heat exchanger has a vent pipe 2 connected to it, the upper orifice of which opens into the ambiency so that when, upon filling up, water enters the heat exchanger, airand other gases may escape from its interior throughthe vent pipe 2 into the ambiency. On'the other hand, with emptying of the heat exchanger air is permitted to penetrate there into through the same vent pipe 2. In operation of the heat exchanger, gases having been absorbed by the cooling water during circulation of the latter collect in the upper water chamber l of the heat exchanger and escape likewise through the vent pipe 2 into the ambiency.

In operation of the heat exchangers, a pressure is to be maintained in the upper water chamber 1 which is only slightlyhigher, than the ambient pressure. Then, the level of cooling water in the vent pipe 2 may occupy a position such as level 3. Generally, the water level 3 will be higher by about 0.5 meter than the highest point of the upper water chamber 1. Upon further heat exchangers being filled up, the pressure prevailing in the water chamberl has to considerably exceed the operational pressure and will have a value of, e.g., 6 meters water column. Then, the water level will be as high as indicated at reference character 9. Thus, the constructional height of the vent pipe 8 has to be selected so that the water level 9 does not reach the upper orifice of the vent pipe 2. Then, the vent pipe 2 will be suitable to discharge air at various pressures without water escaping from the heat exchanger.

In order to prevent freezing of water and of vapours which may stay in the'vent pipe 2, the latter maybe surrounded by a jacket 4, the chamber between vent pipe 2 and jacket 4 being filled with an anti-freeze liquid such as transformer oil. By such anti-freeze liquid heat is transmitted across the wall of the vent pipe 2 from the warm cooling water flowing through the heat exchanger, such heat being carried by natural circulation of the anti-freeze liquid to the upper extremity of the vent pipe 2. Thus, even in portions of the vent pipe 2 remote from the upper water chamber 1 no vapours or t stagnating cooling water will freeze and obstruct the vent pipe.

Where the heat transmission surface between the anti-freeze liquid and the cooling water is small, it is possible to employ heating such as electrical heating 8 in the chamber 5 between the vent pipe 2 and its jacket 4. By means of such external heating sufficient amount of heat can be supplied into chamber 5 to warrant a suitable operation of the breather under optional climatic conditions without running the risk of freezing. Such heating even permits to warm up the cooling water in the vent pipe 2 to its boiling point whereby it is rendered unable to absorb gases although the vent pipe is in connection withthe ambiency. Heat losses can be obviated by employing a heat insulation around the jacket 4. The chamber 5 has a spillway 7 connected to it which prevents that the jacket 4 be damaged by undesired pressures even when the anti-freeze liquid has to be overheated.

FIG. 3 shows the connection diagram of an embodiment with which the breather according to the invention has a plurality ofheat exchangers associated with it. For this purpose, there is the provision of a common vent pipe '2' which is' similar to vent pipe 2 shown in FIG. 2. The vent pipe 2 is, in the instant case, connected to a collecting pipe conduit 13 by which the members of a group of heat exchangers 12 are connected to one another. The collecting pipe conduit 13 is connected to the upper water chambers of the individual heatexchangers 12 and serves for conducting the gases escaping therefrom into the vent pipe 2'. At both extremities of the collecting pipe'conduit 13 the latter is also connected to the circulation line 16 and 14 of the heat exchangers 12. Thus, the cooling water flows in the direction of arrows 10 through the pipe conduit 13 into the lower portion or bottom part of the vent pipe 12 wherefrom it flows through a pipe conduit 15 into the reflux pipe conduit 16 of the circulation system.

An exemplified embodiment of the bottom portion of such vent pipe 2 is illustrated in FIG. 3a. It will be seen that, in the instantcase, a vent pipe 2' isconnected to the collecting pipe conduitlfl which is connected with the upper water chambers 'of the individual heat exchangers 12 and, at the same time, withthe pipe conduit 14 of the circulation system. The cooling water flowing through the system arrives through pipe. conduit 13 and withdrawsthrough pipe conduit 15 which is connected with pipe conduit 16 of the main circulation line in a manner not shown.

The vent pipe 2' is surrounded by a jacket 4 enclosing a chamber therewith. This chamber is filled with an anti-freeze liquid such as oil, and is equipped with external heating means such as electric heater 8. The anti-freeze liquid is kept at a temperature at which no freezing of the various liquids in the breather may occur since the anti-freeze liquid in the chamber between the vent pipe 2' and the jacket 4 is warmed, on the one hand, by the cooling water in the vent pipe 2 and, on the other hand, by the electric heater 8 in such amanner that its temperature is raised beyond its freezing point. Thus, freezing-in of both the collecting pipe conduit 13 and the vent pipe 2 is avoided by circulation. At exceedingly low temperatures heat insulation may be applied to the pipe conduits where necessary.

The exemplified embodiment shown in FIG. 4 is distinguished from the previous ones by that one portion 17 of the vent pipe 2 is of reduced diameter. The length of the portion 17 such that in normal operation of the system the level of the cooling water assumes a position within the portion 17 of reduced diameter. A sphere 18 in vent pipe 2 has a specific weight smaller than the specific weight of the cooling water, the diameter of the sphere being slightly smaller than the diameter of the reduced portion 17 of the vent pipe 2. Thus, the sphere 18 will float on the cooling water surface.

In normal operation of the system the sphere 18 occupies a position within the reduced portion 17 since the level of the cooling water assumes a similar position. The greater part of the cooling water surface is covered by the sphere so that it will not absorb unduly high amounts of air from the ambiency.

Such embodiment will preferably be employed where the cooling water which is the medium of heat exchange must contain soluble gases as is the case, e.g., with the condensation systems of power plants. Upon filling up the heat exchangers a considerable amount of air has to be suddenly removed. Then, the sphere 18 i will be raised by the air pressure from the reduced portion 17 and introduced into a subsequent portion of the vent pipe 'of unreduced diameter so that a suitable free cross-sectional area will be accessible to the escaping air.

On the other hand, in case of quick emptying of the heat exchangers, like in case of danger of frost, the sphere 18 sinks together with the cooling water level into the bottom portion of the vent pipe 2 of unreduced diameter so that, on a sudden, large amounts of air may enter the heat exchangers. The lowermost and The positions of the sphere 18 are determined by built in grids 20 and 19, respectively which are disposed in portions of the vent smaller pipe of unreduced diameter. Moreover, grid 19 prevents the sphere 18 from being removed from the vent pipe 2 by the escaping air flow.

FIG. 5 illustrates an exemplified embodiment with which the vent pipe 2 has, similarly to the previous embodiment,likewise a portion 17 of reduced diameter. The vent pipe 2 is surrounded by a jacket 4 and the chamber between vent pipe 2 and jacket 4 is filled with an anti-freeze liquid as was the case with the previous embodiments. The level of the cooling water will normally assume a position within the reduced portion 17 of the vent pipe 2. With the represented embodiment, the cooling water surface supports a layer consisting of uniform grains such as spheres made of synthetic material and having a diameter of about 4 to 5 millimeters and a specific weight smaller than that of the cooling water. Thereby, it is prevented that air penetrates from above into the cooling water. At transitions from the reduced portion 17 of the vent pipe 2 into its portions of unreduced diameter a pair of vessels 22 and 23, respectively, of porous walls are provided. The reduced portion 17 of the vent pipe and both vessels 22 and 23 encompass a chamber which communicates with the upper and lower portions of the vent pipe 2 of unreduced diameter through the pores of the vessel walls. Thus, upon rising or sinking of the liquid level with re spect to its aforesaid normal position, the grains floating on the cooling water surface are displaced into the vessels 22 or 23 the pores of which are smaller than the grains so that the latter are retained in the vessels 22 or r 7 23 while the cooling water may further rise or sink. The volume of the vessels 22 and 23 is selected so that a suitable cross-sectional flow area is left free for permitting the cooling water or gases to pass through the reduced portion 17 when the grains are entirely within one of the vessels 22 and 23. v

FIG. a shows an operational position of the vessel 23 where the cooling water has already withdrawn from reduced portion 17 and the grains of the material originally floating on the water surface have collected in the bottom part of the vessel 23. Air is permitted to pass through the upper unobstructed pores of the vessel wall to flow in a downward direction as indicated by arrows 24.

Upon filling up the heat exchanger, the grains 21 are carriedby the air flow and liquid flow, respectively, into the upper portion of vessel 22 while a suitable cross-sectional flow area is left free in the bottom part thereof for permitting the withdrawing air to escape.

What we claim is:

1. Breather for a liquid operated heat exchanger of the type comprising, in combination, a heat exchanger having an air outlet, an upright vent pipe connected to the air outlet of said'heat exchanger and opening with its upper orifice into the ambiency, the vertical height of said vent pipe being selected so that the liquid level associated with the highest hydrostatic pressure stays below said upper orifice, a jacket surrounding said vent pipe and enclosing a chamber therewith adapted to be filled with an anti-freeze liquid and to permit a natural flow thereof along the whole height of said vent pipe upon its warming up.

2. A breather as claimed in claim 1, the further improvement of a portion of said vent pipe being arranged for contacting, on the one hand, with the anti-freeze liquid between said jacket and said vent pipe and, on the other hand, with a heat exchanger liquid in said vent pipe, andforming a heat transmission surface adapted to transmit an amount of heat from said heat exchanger liquid into said anti-freeze liquid needed for maintaining the latter at a temperature above its freezing point.

3. In a breather as claimed in claim 1 the further improvement of the provision of a heat source in said chamberbetween said vent pipe and said jacket, and of means for supplying external energy to said heat source.

4. A breather for liquid operated heat exchangers comprising, in combination, a plurality of heat exchangers, each of said heat exchangers having an air inlet and an air outlet, a common collecting pipe con- 8 duit for connecting said plurality of heat exchangers with one another, an upright vent pipe opening with its upper orifice into the ambiency, and connected to said common collecting pipe conduit for'connecting the air outlets of said plurality of heat exchangers with one another, the vertical height of said vent pipe being selected so that the liquid level associated with the highest operational hydrostatic pressure stays below said upper orifice, a jacket surrounding said vent pipe and enclosing a chamber therewith adapted to be filled with an anti-freeze liquid and to permit a natural flow upon warming up of said antifreeze liquid along the whole height of said vent pipe, said collecting pipe conduit being connected to inlets and outlets of the heat exchangers and arranged so as to permit a circulation of liquid therein due to a pressure difference prevailing between said inlets and outlets.

5. In a breather as claimed in claim 4, the further improvement of a portion of reduced diameter being provided between a lower portion of unreduced diameter and an upper portion of unreduced diameter of said vent pipe at a level where the level of a heat exchanger liquid therein associated with normal operation of said plurality of heat exchangers assumes a position in said reduced portion, a sphere being arranged for floating on said liquid level, the specific weight of said sphere being smaller thanthe specific weight of said heat exchanger liquid, and said lower and upper portions of unreduced diameter of said vent pipe being provided with grids the mesh size of which is smaller than the diameter of said sphere.

6. In a breather as claimed in'claim 4 the further improvement of a portion of reduced diameter being provided between a lower portion and an upper portion of unreduced diameter of said vent pipe at a level where the level of a heat exchanger liquid associated with normal operation of said plurality of heat exchangers assumes a position in said reduced portion, said lower portion and said upper portion being connected with one another through vessels having porous walls and enclosing a chamber with one another and with said reduced portion, granular material in said chamber arranged for floating on said liquid level in said vent pipe and having a specific weight smaller than the specific weight of said heat exchanger liquid, and a grain size greater than the size of pore of the vessel walls, the volume of the vessels being selected so that the crosssectional area of the vessel accommodating the entire amount of said granular material is substantially equal to the cross-sectional area of said reduced portion. 

1. Breather for a liquid operated heat exchanger of the type comprising, in cOmbination, a heat exchanger having an air outlet, an upright vent pipe connected to the air outlet of said heat exchanger and opening with its upper orifice into the ambiency, the vertical height of said vent pipe being selected so that the liquid level associated with the highest hydrostatic pressure stays below said upper orifice, a jacket surrounding said vent pipe and enclosing a chamber therewith adapted to be filled with an anti-freeze liquid and to permit a natural flow thereof along the whole height of said vent pipe upon its warming up.
 2. A breather as claimed in claim 1, the further improvement of a portion of said vent pipe being arranged for contacting, on the one hand, with the anti-freeze liquid between said jacket and said vent pipe and, on the other hand, with a heat exchanger liquid in said vent pipe, and forming a heat transmission surface adapted to transmit an amount of heat from said heat exchanger liquid into said anti-freeze liquid needed for maintaining the latter at a temperature above its freezing point.
 3. In a breather as claimed in claim 1 the further improvement of the provision of a heat source in said chamber between said vent pipe and said jacket, and of means for supplying external energy to said heat source.
 4. A breather for liquid operated heat exchangers comprising, in combination, a plurality of heat exchangers, each of said heat exchangers having an air inlet and an air outlet, a common collecting pipe conduit for connecting said plurality of heat exchangers with one another, an upright vent pipe opening with its upper orifice into the ambiency, and connected to said common collecting pipe conduit for connecting the air outlets of said plurality of heat exchangers with one another, the vertical height of said vent pipe being selected so that the liquid level associated with the highest operational hydrostatic pressure stays below said upper orifice, a jacket surrounding said vent pipe and enclosing a chamber therewith adapted to be filled with an anti-freeze liquid and to permit a natural flow upon warming up of said antifreeze liquid along the whole height of said vent pipe, said collecting pipe conduit being connected to inlets and outlets of the heat exchangers and arranged so as to permit a circulation of liquid therein due to a pressure difference prevailing between said inlets and outlets.
 5. In a breather as claimed in claim 4, the further improvement of a portion of reduced diameter being provided between a lower portion of unreduced diameter and an upper portion of unreduced diameter of said vent pipe at a level where the level of a heat exchanger liquid therein associated with normal operation of said plurality of heat exchangers assumes a position in said reduced portion, a sphere being arranged for floating on said liquid level, the specific weight of said sphere being smaller than the specific weight of said heat exchanger liquid, and said lower and upper portions of unreduced diameter of said vent pipe being provided with grids the mesh size of which is smaller than the diameter of said sphere.
 6. In a breather as claimed in claim 4 the further improvement of a portion of reduced diameter being provided between a lower portion and an upper portion of unreduced diameter of said vent pipe at a level where the level of a heat exchanger liquid associated with normal operation of said plurality of heat exchangers assumes a position in said reduced portion, said lower portion and said upper portion being connected with one another through vessels having porous walls and enclosing a chamber with one another and with said reduced portion, granular material in said chamber arranged for floating on said liquid level in said vent pipe and having a specific weight smaller than the specific weight of said heat exchanger liquid, and a grain size greater than the size of pore of the vessel walls, the volume of the vessels being selected so that the cross-sectional area of the vessel accommodating the entire amounT of said granular material is substantially equal to the cross-sectional area of said reduced portion. 